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Exterior roof insulation system with metal roof

Joel Cheely | Posted in Green Building Techniques on

I have a circa 1850 timber frame house near Syracuse NY (zone 5a). We get a lot of snow! This house needs everything – siding, roof, windows so it is ideal for installing exterior insulation. In addition to renovation of all of the exterior I will be constructing an addition. I have a good feel for what I want to do on the addition. The existing roofs are 7/12 and 5/12 slopes. There is one small valley but otherwise are simple gables. Rafters are small and widely spaced. Decking is wide rough-sawn boards in good shape but as is typical for this era house, there are gaps between boards, splits, etc. I have a few questions on the renovations:

1. Is there a chance I can install peel and stick membrane directly to board roof sheathing or should I plan on adding OSB on top of boards? Roofs have lots of slope and membrane will be covered fairly quickly. It seems that one of the lesser grades of peel and stick is ok in this method.

2. I plan to use exposed fastener metal roofing. Per suggested detail I will install 2x4s attached through foam (I plan to use 2 layers 2″ XPS) to create vent space (aligned with rafters). I was going to add 2×4 purlins on top of first layer of 2x4s to attach roof to. I can find no details or photos of roof installed this way. Has this been done successfully?

3. Can I just lap the joints in foam, and tape the joints under the roofing? Felt or some other membrane on top of foam seems like a waste of resources, as taped foam will shed moisture and peel and stick is the last line of defense.

4. I plan to open up ceilings to expose the timber frame where I can. I’ll have to somehow improve the rafters but it may be difficult to add enough depth to put R-30 below the sheathing to get to R-49. It doesn’t seem to matter if I underachieve here, correct? It will be so much better than what it’s replacing (anyone else have vermiculite insulation in their attic?

Enough for now. Thanks in advance for the input!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joel,
    Q. "Is there a chance I can install peel and stick membrane directly to board roof sheathing?

    A. This is a judgment call. The answer depends on whether the sheathing boards are rough-cut or planed, how dusty they are, and the width of the gaps between the boards. If the gaps are narrow, the boards are clean, and the lumber is planed, you are probably good to go. If the situation is iffy, a layer of OSB might be a good idea.

    Q. "I will install 2x4s attached through foam (I plan to use 2 layers 2" XPS) to create vent space (aligned with rafters). I was going to add 2x4 purlins on top of first layer of 2x4s to attach roof to. Has this been done successfully?"

    A. Yes. You can probably use 1x4 purlins if you want.

    Q. "Can I just lap the joints in foam, and tape the joints under the roofing?"

    A. I don't understand this question. How can you lap the joints in the rigid foam? I think you will need to butt the panels together. Of course, you can choose rigid foam with tongue-and-groove joints if you want.

    Q. "Felt or some other membrane on top of foam seems like a waste of resources, as taped foam will shed moisture and peel and stick is the last line of defense."

    A. I'm a believer in the use of roofing underlayment in all cases. Rigid foam can shrink; if this happens, tape might fail, and water would get between the cracks. While the rubberized membrane is, indeed, a last line of defense, why not include the roofing underlayment? It doesn't cost much; it's a code requirement; and it is required by the installation instructions of most roofing manufacturers.

    Q. "It may be difficult to add enough depth to put R-30 below the sheathing to get to R-49. It doesn't seem to matter if I underachieve here, correct?"

    A. The answer depends on your performance goals. If you install less than the code-minimum requirements for roof insulation, your roof will leak heat at a faster rate than a code-minimum house. I urge you to contact your local building department in any case, to make sure that your plans comply with local code requirements.

  2. Joel Cheely | | #2

    Thanks for the input. Boards are rough cut and gaps up to 5/8" so new layer of OSB it is. 1x4 purlins won't work as rafters are 36" +/- on center. Sorry I sounded confusing on "lap joints"; I meant staggered joints for two layers of insulation.

    I agree felt is cheap but where would I add it and how? There's no way to attach to foam that I know of. It seems to call for another layer of sheathing in lieu of purlins so I can install felt in a code-approved manner. I like the ladder effect of purlins as it makes metal roof install easier. Again, has anyone done this installation method with metal roof or is this theory?

    Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    Joel,
    To install roofing underlayment on top of rigid foam, use cap nails. All you need are enough cap nails to hold the underlayment in place until the purlins are installed.

    You can buy cap nails in lengths up to 8 inches:
    http://www.simplexnails.com/InsulationCapDataSheet.html

  4. Brad Hardie | | #4

    Joel,

    Lots of houses are built with this method. Look to the the Cold Climate Housing Research Center (CCHRC) for info on it, there are even videos.

    http://www.cchrc.org/retrofits
    or here are the videos
    Part 1 plays right into Part 2 or there are separate links

    REMOTE - Walls - Part 1
    https://www.youtube.com/watch?v=1leyQtqVV-A
    and
    REMOTE - Walls - Part 2
    https://www.youtube.com/watch?v=9GIrSF26SjA

    I'm doing my house and barn this way (only I have 6 layers on the house and 5 on the barn). On the barn to reduce the weight (I've got exposed rafters and barn board beneath) - I put down luan (1/4" sheathing). I stapled it down to the barnboard, so that the ice & water shield I used doesn't show through the knots and gaps.

    I also went with just the IKO ArmourGard (ice & water shield) instead of the Grace Ice & Water Shield, because it was less than half the cost, and because it is going to be covered in LOTS of other layers of rigid insulation, and purlins, and more sheathing and then standing seam metal roof. If I get bulk water all the way down to the ice & water shield.....I've got BIG problems. The less expensive stuff is not as flexible as Grace, but I only used it on the pitched roof, valleys and hips. Everywhere else I used liquid flashing by Prosoco - R-Guard liquid applied flashings and membranes.

    For the longest time, I used nothing but Grace, but a few years ago, I was doing a remodel that had used the IKO ArmourGard, and it was so resilient still I have used it on other jobs in areas where the flexibility isn't a concern.

  5. Joel Cheely | | #5

    Brad, thanks for the input. Did you run two layers of purlins (one with the slope and one parallel)? Since the air space isn't really a true ventilation channel, and there are no good ventilated metal roofing ridge details (I've designed a few that were head scratchers), I'm thinking of just installing one layer of purlins parallel to the ridge. The metal roof will have some isolation from the insulation and the air space at each rib will provide some opportunity for air movement.

  6. Brad Hardie | | #6

    Joel,

    Honestly, I'm not quite there. Just like many others have done though - I'll have purlins (which the screws go through back to the framing), and 5/8 sheathing on top of that, then a roofing membrane (not ice and water (except at the first 3 feet)), because even on a cold roof, if the soffit overhang (eave) isn't insulated, the hot air that builds up under the soffit from convective currents can still cause ice damming. I'll then have standing seam metal roof atop of it all.....

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